1. Field of the Invention
The present invention relates generally to cordless telephones and, more particularly, is directed to a cordless telephone of a multi-channel access (MCA) type in which a master station includes a plurality of sets of transmitting and receiving circuits and communication channels and simultaneously opened between the master station and a plurality of remote stations or handset units.
2. Description of the Prior Art
A low power-type cordless telephone has been provided in Japan and employs a multi-channel access (MCA) system in which 87 communication channels and 2 control channels are provided. In order to expand the functions of such cordless telephone, a so-called multi-cordless telephone has been proposed, in which one master station having a plurality of base units, each containing a respective set of transmitting and receiving circuits, is combined with a plurality of remote stations or handset units. In such multi-cordless telephone, communication channels may be simultaneously opened between the master station and a plurality of the associated remote stations. For this purpose, a microcomputer in the master station causes an unoccupied receiving circuit to sequentially scan the communication channels and provides data indicating vacant channels and which are stored in a memory as vacant channel information. When communication channels are opened between the master station and the remote stations, a communication channel indicated to be vacant by the stored vacant channel information is employed for each channel. However, when the master station simultaneously transmits and receives radio waves on more than two communication channels, problems may arise due to cross-modulation even though each communication channel employed was indicated to be a vacant channel. Such cross-modulation can affect the quality of the telephone communication as well as the response to commands transmitted via the control channel or channels.
So long as the sets of transmitting and receiving circuits are situated proximate to each other in the master station, as is desirable, or the remote stations or handset units are operated simultaneously, the cross-modulation disturbances cannot be suppressed provided that the frequencies of the opened communication channels are such as to induce the cross-modulation.